The present invention relates to valved cryogenic refrigerators, in particular, Gifford McMahon (GM) refrigerators, and GM type pulse tube refrigerators. Gas is cycled between high and low pressures by a valve mechanism that connects to an expander. The valve mechanism commonly consists of a rotary valve disc and a valve seat. Rotary disc valves lend themselves to being designed with multiple ports. There are discrete ports, which, by periodic alignment of the different ports, allow the passage of a working fluid, supplied by a compressor, to and from the regenerators and working volumes of the expander.
GM and Solvay type refrigerators use compressors that supply gas at a nearly constant high pressure and receive gas at a nearly constant low pressure. The gas is supplied to a reciprocating expander that runs at a low speed relative to the compressor by virtue of the valve mechanism that alternately lets gas in and out of the expander.
W. E. Gifford also conceived of an expander that replaced the solid displacer with a gas displacer and called it a “pulse tube” refrigerator. This was first described in U.S. Pat. No. 3,237,421 which shows a pulse tube connected to valves like the earlier GM refrigerators.
Early pulse tube refrigerators were not efficient enough to compete with GM type refrigerators. A significant improvement was made by Mikulin et al., as reported in 1984, and increased interest ensued in searching for further improvements. Descriptions of major improvements since 1984 can be found in the references listed herein. All of these pulse tubes can run as GM type expanders that use valves to cycle gas in and out of the pulse tube. GM type pulse tubes running at low speed are typically used for applications below about 20 K.
This type of valved cryogenic refrigerator has the disadvantage of low efficiency due to the pressurization and depressurization of the void volumes in the expander as gas cycles in and out of the expander. In a valved cryogenic refrigerator, there is a large pressure difference through the high pressure valve right after it opens, because the pressure at the inlet of the regenerator is near the low pressure. On the other hand, when the low pressure valve opens, there is also a large pressure difference through the valve, because the pressure at the inlet of the regenerator is near the high pressure. This process generates an irreversible loss which cannot be decreased by enlarging the opening area of the valves. The loss pertains to the void volume of the cold head.
In Japanese patent P2001-317827 to Fujimoto, two buffers are connected to the inlet of the regenerator by two rotary valves controlled by a timing sequence as shown in FIG. 2 of P2001-317827. In this patent, during charging process, in the first step, gas first flows into the regenerator from the first buffer. In the second step, gas flows from the supply side of compressor into both the regenerator and the first buffer. The effect of the extra first buffer shown in this patent is small since the amount of gas which flows into the regenerator from the first buffer in the first step has to be compensated from the compressor in the second step. During discharging process, in the third step, gas flows out of the regenerator into the second buffer. In the fourth step, gas flows from both the regenerator and the second buffer to the return side of compressor. The effect of the extra second buffer shown in this patent is small since the gas which flows into the second buffer from the regenerator in the third step has to flow out of the second buffer into the compressor in the fourth step.
It is an object of this invention to reduce the amount of gas supplied by the compressor and to provide a cryogenic refrigerator with reduced pressure drop during gas cycling.